This invention relates to transmitting data from a transmitting terminal to a receiving terminal through a third component, and more particularly to transmitting video data for videoconferencing and video telephony, and error correction and concealment during such transmission.
Transmitting data by a packet-switched network is one of the most common methods of transmitting data. As with any other type of data transmission, data transmitted by a packet-switched network can be affected by transmission errors such as loss of packets.
Loss of some packets in a data flow can significantly affect the rest of the data flow. For example, consider the situation where the transmitted data is digital video encoded by a prediction based compression technique. In that case, loss of packets will affect not only a particular frame to which the data in the lost packets belong, but also subsequent frames. In addition, if the compression technique uses motion compensation, then the lost packets will affect not only a particular region in the frames, but also surrounding regions in the subsequent frames, the extent of which depends on the value of the motion vectors. Similarly, if the transmitted data is digital video encoded using variable length coding (for example, Huffman coding), the packet loss can render the information contained in one or more of the subsequent packets unusable.
Various techniques have been developed to minimize, and even correct for, the effects of packet loss on transmitted video data. One set of techniques attempt to reduce the effect of packet loss by including redundant control data in all packets. For example, some packetization protocols require control data necessary for decoding a packet to be included in a packet""s header, even though the same information is included in a preceding packet.
Another set of techniques attempt to reduce the effects of lost data on the video image by replacing the lost data with other data. For example, according to one such technique, the lost data is replaced with data from a preceding frame, thereby attempting to improve the image quality of the current frame and reduce errors in subsequently decoded frames. Yet another set of techniques provide methodologies for allowing a receiving terminal to determine whether a packet has been lost and, if so, send a request for a correction of the lost data to the transmitting terminal. The transmitting terminal then provides data which corrects the effects of the lost data. For a summary of various error concealment and correction techniques, see Y. Wang and Q. -F. Zhu, xe2x80x9cError Control and Concealment for Video Communication: A Review,xe2x80x9d Proc. IEEE, vol. 86, no. 5, pp. 974-997, May 1998. In another technique, a network switch detects a packet loss and requests a retransmission of the lost packet. The switch does not forward subsequent packets until it receives the replacement packet. Of course, if the packets contain video data being transmitted for realtime display, the delay can have significant negative impact on the displayed video.
It should be noted some of the these techniques require that a receiving terminal detect the packet loss. However, error detection at the receiving terminal becomes difficult where the data transmission path includes two networks governed by different protocols because a gateway between the two networks typically removes packet headers from the packets before forwarding the information contained in the packets to the receiving terminal. Removal of the packet headers makes error detection more difficult, since the packet headers typically contain data which can be used for error detection. One type of error detection that a receiving terminal can perform is to use its video decoder to detect packet loss errors by checking whether the received video bitstream is in conformance with the bitstream syntax of the encoding algorithm.
In a general aspect, the invention relates to transmitting data from a transmitting terminal to a receiving terminal through a server. The invention features detecting transmission errors in the data at the server and, if an error is detected at the server, sending a request from the server to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error. The affected data is transmitted from the server to the receiving terminal prior to receiving the correction data.
In another general aspect, the invention relates to a server for receiving data transmitted by a transmitting terminal and transmitting the data to a receiving terminal. The server features a receiver receiving data transmitted from the transmitting terminal to the server; a detector detecting transmission errors in the data; a first transmitter sending, if an error is detected, a request to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error; and a second transmitter transmitting the affected data from the server to the receiving terminal prior to receiving the correction data.
In yet another aspect, the invention relates to a system including a transmitting terminal, a receiving terminal, and a server, where the transmitting terminal transmits the data to the server and the server transmits the data to the receiving terminal. The invention features a server which detects transmission errors in the data, and, in response to certain of such errors, sends a request to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error. The server sends the affected data to the receiving terminal prior to receiving the correction data.
Preferred embodiments of the invention may include one or more of the following features.
The server is connected between two networks transmitting data from one network to another. The data includes a plurality of packets. Data can be digital video or compressed digital video, such as video compressed by a compression technique which uses motion compensation.
The transmission error can be loss of a packet. Where the transmitted packets are consecutive packets of data identified by sequence numbers, the loss of the packet can be detected by identifying a missing sequence number. The server determines the pixels of at least one frame of video affected by the lost packet, and includes in the request sent to the transmitting terminal information identifying at least a portion of the pixels to the transmitting terminal. The pixels can be from a single frame or from more than one frame.
The data received at the server is processed to conceal the detected error from the receiving terminal or other downstream components. Where the data is transmitted by the server to the receiving terminal according to a predetermined syntax, the process of concealing the error includes modifying the packets such that the transmitted data conforms to that predetermined syntax.
In response to the request, data can be sent to the receiving terminal to correct the data affected by the transmission error. Where the data is digital video data compressed by a prediction based compression technique, and some of the data is lost, the digital video data can be compressed, without relying on video data corresponding to data lost during transmission, to provide the data to be sent to correct the data affected by the transmission error.
The invention may include one or more of the following advantages.
Embodiments of the invention reduce the time required to recover from transmission errors such as packet loss when transmitting data (for example, video) between two or more terminals. In addition, some embodiments improve end-to-end video quality when compressed video data is lost or damaged during transmission.
Where the transmitting terminal is multi-casting to multiple receiving terminals through a single server, by detecting the error at the server rather than at each of the receiving terminals, only a single error correction request is sent to the transmitting terminal as opposed to multiple requests from the receiving terminals.
In addition, by concealing the transmission error from the receiving terminals, picture quality at those terminals can be improved as those terminals need not wait for response to their error correction requests. Waiting for such a response can result in the terminal freezing the image on the monitor for a relatively long time. In addition, concealing the error from downstream components and receiving terminals reduces the possibility of receiving multiple error correction requests from those terminals.
The invention may be implemented in hardware or software, or a combination of both. Preferably, the technique is implemented in computer programs executing on programmable computers that each include a processor, and a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements). Program code is applied to data entered using the input device to perform the functions described above and to generate output information. The output information is applied to one or more output devices.
Each program is preferably implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language.
Each such computer program is preferably stored on a storage medium or device (e.g., ROM or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer to perform the procedures described in this document. The system may also be considered to be implemented as a computer-readable storage medium configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner.